Electron focusing structure



y 1, 1957 E. o. LAWRENCE 2,793,317

ELECTRON FOCUSING STRUCTURE Filed Oct. 22, 1954 2 Sheets-Sheet l May 21,1957 E. o. LAWRENCE ELECTRON FOCUSING STRUCTURE 2 Sheets-Sheet 2 FiledOct. 22, 1954 INVENTOR. femur 0. Zia/25w:

vswri IITOiAIYJ 2,793,317 ELECTRON FocUsING STRUCTURE Ernest 0.Lawrence, Berkeley, Calif assignor to Chromatic Television Laboratories,Inc., New York, N. Y., a corporation of California Application October22, 1954, Serial No. 464,141 9 Claims. (Cl. 315-2 1 This inventionrelates to post-deflection focusing structures for use in cathode-raytubes intended for displaying television images in color, thisapplication being a continuation-in-part of copending' applicationSerial No. 219,- 213, filed April 4, 1951, now Patent No. 2,692,532, andentitled Cathode Ray Focusing Apparatus.

The structures particularly to be considered in the present applicationare intended for use in determining the hue to be displayed on atelevision screen which comprises groups of a plurality of phosphorsemissive on electron impact of light of different colors which areadditive to produce white, arranged in a repeating pattern over thesurface of the screen. Each group includes at least one phosphor of eachof the different ones used, these usually being three, emissive,respectively, of red, green, and blue light, although this is not theonly combination which may be used. The dimension of each group, in atleast one direction, is of the order of magnitude of one elemental areaor picture point of the image to be produced on the screen, so that eachphosphor component of the group is sub-elemental in dimension. In,operation of the tube the screen is scanned bi-dirnensionally by anelectron beam or beams so as to trace a raster thereon. If a pluralityof beams are used they are converged so as to impact the screen at ornearly at the same, point, all of the beams falling, at any giveninstant, within an area of picture-point size.

The cross-section of the electron beam as it approaches the target area,within which the screen lies, is also of the order of magnitude of onepicture element, so that if all of the electrons reached the screenfollowing undeflected paths phosphors of all of the component colorswould be exicited and the effect of white light would be produced.

In the application of which this is a continuation-inpart there areshown a large number of electrode structures which, mounted in front ofthe screen, form a multiplicity of electron lenses which converge theelectrons forming a beam into a focal spot of sub-elemental size,whereby, at any instant, the electrons of a beam will impact only aphosphor emissive of light of the desired color. If a single beam isused means are provided for deflecting it, at will, onto the desiredphosphor; if a plurality of beams are used, their angles of incidence atthe screen are slightly different and they are sodirected as to fall,respectively, upon the different phosphors, the color displayed by thetube, at any instant, being determined by modulating the beamsindividually so that light of the proper color composition is emitted. VV

In one form of screen, to which the present specification isparticularly directed, the phosphor pattern is comprised of narrowstrips of the respective phosphors, and the apertures of the electronlenses are defined by elongated linear conductors, stretchedsubstantially parallel to the phosphor strips. The positioning of theconductors with respect to the pattern on the screen is such that eachaperture, formed between adjacent conductors, is electronopticallyalined with a corresponding group of phosphor strips.

, 2,793,317 Patented lVlay 21, 1957 In order to constitute electronlenses, there must be associated with the structure thus described atleast one additional electrode which is electron-permeable; i. e'.,through which electrons can pass as they do through the apertures of thegrid formed by the elongated conductors. When different potentials areapplied to the grid and to the electron-permeable electrode orelectrodes, electric fields are set up between the two which deflect theelectrons of the beam, these fields constitutingin fact the electronlenses through which focusing is accomplished. The electron-permeableelectrodes through which, in conjunction with the grid electrodes, thesefields are established may take several forms; they may themselves'be ofgridlilce structure, they may be formed of metal gauze, or they may bethin metallic films, the latter usually being the case where theelectron-permeable electrode lies directly upon the display screenitself. The nature of the lens formed by the combination depends on boththe positioning of all of the electrode elements (hereinafter, forconvenience, sometimes referred to as lens elements, even though thefields themselves constitute the actual lens) and upon the ratio of thepotentials of those elements with respect to the potential of thecathode from which the electrons are emitted and with respect to eachother. By altering these potentials and the positioning of theelectrodes the same structures may be either converging or diverginglenses, their focal lengths may be changed, and the points on thescreens at which the various apertures focus may be altered.

Numerous electrode structures for forming cylindrical electron lenses ofthe character described' are shown and described in the applicationabove referred to; broadly speaking these structures are generallyequivalent, and the choice as to which structure is to be employedbecomes largely a question of mechanical convenience. Under certaincircumstances, however, certain arrangements have material advantagesnot possessed by the others. This specification refers broadly to anarrangement wherein there is mounted on at least one side of the gridwhich defines the lens apertures, an electron-permeable electrode, quiteclosely spaced in relation to the grid, and comprises conductingelements lying within the grid apertures as viewed along the electronpath; preferably transverse to the grid conductors. With thisarrangement there may or there may not .be also used an additionalelectron-permeable electrode in the form of a conductive film on thescreen surface. In operation, electron-permeable electrode close to thegrid is operated at a potential positive with respect to the grid, whilethe screen surface operates at a potential which is nearly the same asthat of the lens element nearest it, but preferably slightlynegativethereto;

Among the objects of the structure thus described are: to provide afocusing structure which will produce minimum distortion of the field'of view; to provide a focusing structure which is the analog of a thinlens, and wherein the electron paths from the lens apertures to thescreen canbe made substantially rectilinear; to provide a lens gridstructure wherein the spacings of the lens elements and the phosphorstrips canbe made uniform" throughout the area of the viewing field; toprovide focusing structure wherein relatively small potentialdifferences can be used to achieve the required focusing effect; toprovide a focusing structure which can be operated at potentials such asto minimize halation caused by spurious electrons falling on the screen;and to provide a lens grid structure which is particularly applicable todisplay tubes using the angle of incidence of electrons at the structureeither from a plurality of electron guns or from a single gun whose beamis deflected to change the angle to control the color displayed.

In the accompanying drawings, to which reference will 'be made toexplain the principles of operation of the present invention:

Fig. 1 is a schematic diagram of a cathode-ray dis- :play tube embodyingthe invention, the tube shown being Fig. 3 is a cross-sectionalview-through a portion of the target structure of a tube, showing therelationship between the grid, electron-permeable electrodes and screenelements, as employed in one embodiment of the invention, thisembodiment being one of those used to produce the analog of cylindricaloptical lenses of the doubleconvex type;

Fig. 4 is a similar diagram illustrating another embodiment of a doubleconvex lens;

Fig. 5 is a fragmentary sectional view, similar in type to Figs. 3 and4, illustrating a structure which is the elecdicular wires, andillustrating diagrammatically the shape -of the electric fields whichaccomplish the focusing, the

plane of section being normal to the wires of the aperture grid; 7 7

Figs. 7A and 7B are cross-sectional diagrams through .the structure ofFig. 6, the plane of section in both cases being normal to that of Fig.6. Fig. 7A illustrates the .relationships of the elements at the centerof the screen while Fig. 7B is a showing of the same relationships asthey appear at the edges of the screen;

Fig. 8 is a showing of a structure superficially somewhat similar tothat of the present invention but which develops fields of a differenttype;

Fig. 9 shows the fields established by the same structure that isillustrated in Fig. 8 when the relative potentials of a grid andelectron permeable elements are reversed; and

Fig/ 10 illustrates a structure of electrodes similar to that shownin'Figs. 8 and 9 but with the position of the 'conductors'of the gridsshifted with respect to that of the two preceding figures, soas toaccomplish a concentration of the electron beam which is not, however, atrue focusing action, but which is an electron-optical analog of anaggregate of prisms.

With the exception of the target structure to which the presentinvention particularly relates, the elements of the tube, shownschematically in Fig. 1, are conventional. The tube comprises evacuatedenvelope 1, which is generally funnel-shaped and the body of which maybe either of metal or glass. In the glass neck of the funnel threeelectron guns are mounted, each comprising a cathode 3, a grid 5, andone or more anodes 7. Individually the guns are conventional. Eachproduces a narrow beam of electrons converging to a common focal point9. Electrons of the beam passing through this focal point arereconverged by a focusing coil included in a yoke 11 at a conjugatefocus in the target area, generally defined by a window 13 which closesa larger end of the envelope. In accordance with conventional practicethe yoke 11 also includes coils for deflecting the three beamsconcurrently and bidimensionally so as to trace a raster upon thedisplay screen which occupies the target area. Except for the fact thatthe three electron guns are mounted side by side instead of clustered ina triangle, this construction is substantially conventional.

In the present instance, the fluorescent surface which comprises theviewing screen is deposited directly upon the window 13. The nature ofthis pattern is best illustrated in Fig. 2. It comprises repeatinggroups of strips of phosphors 15x, 15g and 15b, emissive upon electronimpact of red, green, and blue light respectively. If the diameters ofthe electron beams as they enter the target area are in the neighborhoodof 30 mils in diameter, the width of each group of strips should be nolarger than this, and the individual strips will each be in theneighborhood of 10 mils wide, each strip extending across the entireviewing area. So far as the present invention is concerned it makes nodifference whether the strips run vertically or horizontally as the tubeis viewed when in operation.

Preferably, although not necessarily, a thin conductive film 17, usuallyaluminum, is deposited on the surface of the phosphor strips; The filmmay be connected to the envelope 1 if it is of metal, the conductingcoating conventionally deposited upon the inner surface of the envelopeif it be of glass, or a film lead 19 may be brought out through theenvelope so that an external source of potential may be connecteddirectly to the film and it may be operated at a potential differentfrom that of the envelope itself.

The lens structure for focusing electrons so that they will impinge uponthe specific phosphor which is emissive of the desired color is mountedadjacent to the screen and generally parallel thereto, the termgenerally parallel as used herein meaning that the corners of thestructures so' designated are equidistant. It is to be understood thatthe screen, if deposited upon the window of the tube, will ordinarily besomewhat curved in one or both dimensions, but it is also to beunderstood that the screen may be deposited upon a'plane base, mountedentirely within the tube and occupying, generally, the target area. In

practice the spacing between the grid and screen will usually be in therange between 0.40 inch and 1 inch, but these are not limiting values.

In its preferred form, the electrode structure which establishes thefields for focusing the electrons comprises a grid 21 of tightlystretched, elongated, linear conductors, which may be either narrowmetal tapes or wires. If tapes, they are mounted substantially edge-onwith respect to the electron paths, so as to offer minimum obstructionto the passage of electrons between them. The tape formation may bepreferred with tubes using 'microdeflection of a single beam for colorcontrol. With a three gun tube of the type here illustrated, however,the use of wires is ordinarily convenient for mechanical reasons; as faras the electron lens action is concerned, tapes and wires areequivalents. V

The positions of the conductors constituting the grid. with respect tothe groups of phosphor strips, are illustrated in Fig. 2. As has alreadybeen indicated this figure shows the actual physical position of thegrid conductors as viewed at the center of the screen. If it werepossible to view the screen and the" grid along the paths of theelectrons, it would represent the relative positions in all parts of thescreen, but due to the refraction of the beam in passing through theelectron lens structure, the relative positions might be different,although only slightly different, than if viewed optically from thecenter of deflection of the beams as'they are scanned over the targetarea. Electron-optically, therefore, the conductors 21 of the grid areeach centered over a junction between successive groups of phosphors; inthe case shown, a junction between a red and a blue phosphor stripe Inthe pattern illustrated in this figure, wherein there are an equalnumber of strips of each phosphor in the over-all pattern, the junc tioncould justas well be between a red and a green strip, or a green and ablue strip.

Positioned closely adjacent to the grid 21, generally parallel. theretoand preferably equally spaced on each side thereof, areelectron-permeable electrodes 23 and 25. These electrodes can be eitherof a fine metal gauze, as illustratedin Figs. 1 and 3, or they maycomprise tightly stretched, fine wires. Y

The essential feature of these electrodes, if they are to exercise thefunction which gives this type of focusing structure its particularadvantages, is that they must inelude conductive elements lying withinthe apertures defined by the grid, and preferably centered within theapertimes as viewed along the electron paths entering them. Ideally, thetwo electrodes 23 and 25 should approach, as nearly as possible,equi-potential planes, so that the lines of force which terminate uponthe grid conductors will terminate on elecrodes 23 and 25 in asubstantially equal distribution thereacross. Practically, it would bedifficult, if not impossible, to meet this ideal situation in astructure which could be physically realized. If the electrodes 23 and25 are to be self-supporting, the conducting elements which comprisethem must be of finite size. This implies that these electrodesthemselves must be apertured and their physical embodiments must includeconductors upon which the lines of force of the electric field willconcentrate. Such a concentration of field means, in turn, that eachaperture formed between the conductors becomes the aperture of anelectron lens, which will have a diverging eifect upon the electron beamwhen the field at the grid apertures has a converging effect. Thesmaller the aperture the fewer will be the lines of force from theconductors defining each one and hence the less will be the effect ofthe fields upon the electron beam. The smaller the apertures, however,the greater will be the number of electrodes required to form them, andhence the greater will be the proportion of the beam electrons which areintercepted by the conductors, and therefore the less efiicient will bethe electrode structure. One of the features of the present invention,in its preferred form, is that it permits the use of electronpermeab'leelectrodes of extremely open character without introducing divergingeffects which are noticeably deleterious with respect to the imageproduced by the tube.

The electron lens structure here illustrated is operated withelectrodes23 and 25 at the same or nearly the same potential, and with the grid 21negative with respect to both. With this arrangement, as will be shownhereinafter, effect of the fields between the various elements of theelectron lens structure is to deflect the electrons in a directionnormal to the conductors of the various elements. Electrons of a beampassing through electrodes 23 and 25 are diverged or defocused by thefields set up thereby along the phosphor strips, whereas electronspassing between the grid electrodes tend to converge onto a singlestrip. This latter is the desired effect, and by malting the aperturesin electrodes 23 and 25 much smaller than those between the gridconductors, the converging effect completely dominates the divergenceand the lens exercises its desired function. A fine meshed, metal gauze,therefore forms a very satisfactory lens of the type herein considered.If it be assumed, however, that such a gauze is formed of a square mesh,with certain of the conductors running parallel to those of the grid andothers running transverse thereto, it will be recognized that the formerintroduce a partially neutralizing effect, increasing the focal lengthof the desired lens, this defocusing efiect becoming less and less asthe number of conductors running parallel to the grid conductors isincreased. The transverse conductors have no effect on the desired linefocus. Neither does divergence in this dimension change the colordisplayed by the tube, since the electrons of the .beam are merelyshifted along a phosphor strip emissive of the same color. Furthermore,as will be shown, with a suitable disposition of the two electrodes 23and 25, the eifect of the deflections produced by the transverseelements can be very largely compensated. The longitudinal elements ofthe mesh can therefore be eliminated entirely, while those extendingtransversely of the grid apertures can be reduced in number.

A structure embodying such construction leads to a disposition of theconductors of electrodes 23 and 25 such as is illustrated in Fig. 2. Thespacing of these conductors, which are numbered in the figures to correpnd h fe nce h rac er a l d *9 he spective electrodes as a whgle, doesnotnecessarily bear any direct relationship to that of the gridelectrodes, although preferably it should be closer to reduce the sizeof the diverging apertures. It will be noted, that as viewed along theelectron paths conductors 23 are spaced midway between conductors 25. Itwill further be noted that they are materially smaller in diameter. in apractical tube the grid conductors 21 may be 4 to 6 mils in diameter,whereas the conductors 23 and 25 may be as small as 1 mil or even less.This is possible because conductors 23 and 25 are operated at apotential positive to the grid and therefore much heavier fieldconcentrations may be permitted without danger of breakdown due to fieldemission of electrons from the smaller conductors.

fnFig. 6 there is shown a fragmentary cross-sectional view of theconductors and screen, with the plane of section. normal to the gridconductors. In this figure the curved lines, bearing arrows, representroughly the shape of the electric fields between the grid and theelectrodes 23 and 25. The dotted lines 31 indicate electron pathsthrough one of the apertures of the grid. The relative potentialsapplied to the electrodes of the structure are also indicated.

Consider an electron entering near the left edge of the aperture betweena pair of the conductors 21. It will be seen that it will out nearly allof the lines of force from conductor 21 to electrode 23 as it passesthrough the upper half of the structure. The result is a retardation ordeceleration in this region, plus a lateral acceleration which gives ita component of velocity away from the conductor 21 and transverse to itsoriginal path. Passing on, beyond the conductor 21 it will bereaccelerated in the original direction to its original total velocitybut with an additional transverse component in the same direction asthat received in passing through the first region. An electron passingthrough the structure near the right of the aperture and the samedistance from a conductor 21 will be subjected to equal increments anddecrements of. velocity in the longitudinal direction but its transverseacceleration will be in the opposite direction, away from the nearerconductor 21. Electrons passin through the exact center of the aperturewill, however, receive no lateral acceleration. The beam will betherefore converged in passing through the lens structure. Thepotentials applied to conductors 23 and 25 are here assumed to be equal,and if they are equally spaced from the grid, the electrons passingthrough the center of the aperture will emerge from the structure withtheir original velocity and traveling in the same direc tion as theywere when they entered it, although slightly refracted away from thetube axis.

What happens after the electrons emergefrom electrode 25 depends to alarge degree upon the relative potential of the latter electroderelative to the surface of the screen. If the latter consists ofphosphors uncoated with a film 17, secondary electrons will be emittedfrom the phosphor surface, depending in number ,upon the velocity withwhich the beams .strike the screen, and the screen will acquire anequilibrium potential which is within a relatively few volts of that ;ofthe electrodes 25. :If, as is preferable, the film ;17 is present, thenumber of secondary electrons emitted-will begreatly reduced, .thosewhich return to the screen will have lower velocities so that they donot penetrate the film, and, moreover, the relative potential of thesurface can be established at will. With no potential difference appliedelectrons will follow rectilinear paths between :the lens structure andthe ,screen. If there is a potential difference between electrode 25 andthe screen, any electrons which are approaching the screen at an anglewill follow parabolic paths. As such tubes are operated, however, thevelocity in the lens-screen region ishighand the deviation fromrectilinearity -is very slight unless the potential- V 7 difierence isquite large in comparison to the total accelerating voltage between'thecathodes and the screen. It is preferable, therefore, to operate withthe film 17 at approximately the same potential as the screen. Byvarying the potentials of the electrodes of the screen slightly, slighterrors in grid-screen alinement can be compensated.

In the above discussion the deviation of the electrons from theirstraight-line paths as they pass through the focusing structure itselfhas been neglected. The closer electrodes 23 and 25 are together, theless such deviation will be. Furthermore, the closer they are together,the less will be the potential difference required between them and thegrid 21 in order to establish a given num ber of lines of force andachieve a given degree of focus- On the other hand, the more distantelectrodes 23 and 25 are from the grid, the more uniform will be thedistribution of field along the lengths of the transverse conductors andtherefore the more nearly will the deflection of the electrons beproportional to their distance from the center of the aperture. A verysatisfactory result is obtained, however, if the separation of theelectron-permeable electrodes from the grid is equal to the spacing ofthe grid conductors, although some slight improvement in this respectcan be obtained by greater spacing.

Figs. 7A and 7B illustrate the defocusing eflect of the transverseconductors, the plane of section in these figures being normal to thatof Fig. 6. Fig. 7A shows the relative position of the conductors at thecenter of the screen, while Fig. 78 illustrates their relative positionat the edge. 'It will be seen that the arrangement is such that anelectron entering near one side of an aperture in the elect-rode 23 willpass near the opposite side of an aperture in electrode 25, and that theacceleration normal to these electrodes, and therefore parallel to thephosphor strips, is in opposite direction and therefore tends to cancelout. Such cancellation is not complete, however. An electron passingthrough electrode 23 closely adjacent to the center of the aperture-willbe subject to little or no deviation, whereas it will pass close to theedge of an aperture in electrode 25 and be subjected to maximumdeviation. The maximum deviation along any electron path, however, willonly be approximately one-half of what it would 'be if the conductorswere alined parallel to the path of the rays. Both electrodes have adiverging effect, but the effect of electrode 25 will be the lesser ofthe two owing to the fact that it is closer to the screen. With theelectrode conductors staggered as shown the refraction will be morenearly analogous to that of a pair of prisms than to that of a lens, thebeam being split into two.

If conductors 23 and 25 were spaced equally to the grid conductors andwere alined, the spreading of the beam in the direction of the phosphorstrips would be equal to their degree of concentration transversely ofthe strips. If they were still equal in number but staggered, as shown,the amount of dispersion would be cut roughly in half. The amount ofdispersion is, however, directly proportional to the size of theapertures, and by combining closer spacing with the staggeredrelationship the amount of divergence can be made to have negligibleeffect. Some slight divergence is not at all disadvantageous. 'It isfrequently desirable to damp possible vibration of the conductorscomprising the grid 21, and a very effective way of doing this is tothread damp rods, comprising glass rods of a diameter approaching thatof the conductors of the grid, transversely across them. Slightdivergence of the beam serves to eliminate the shadows of such damprods, as well as to eliminate any shadows resulting from the electrodesthemselves.

As canbe seen from the examination of Fig. 7B, where thefjelectrons ofthe beam enter the electron lens structure at nan angle,.the.fields towhichthe electrons are subjected are in the same direction as in thecenter of the screen, where the electron paths are normal to'the lensstructure. In order to accomplish this, the spacing of the conductors 25must be greater than that of conductors 23. The spacings of the two setsof conductors should be substantially proportional to the distances ofthe respective electrodes from the center of deflection of the electronbeam.

It has elsewhere been shown that such a linear relationship does nothold with electron lenses formed between a grid of conductors and aconductive film such as 17 deposited on the screen, with the filmoperated at a potential positive to the grid conductors. Using such atwo-- element lens system, all electrons are deviated toward the axis ofthe tube (as defined by the perpendicular from the center of deflectionto the screen), and if the grid electrodes are uniformly spaced thestrips of phosphor must be slightly narrower at the edges of the screenthan at the center. It has also been shown that this correction forrefraction is directlyproportional to the distance between thelens-f'orming elements. The change in refraction of the principle ray,passing through the center of each aperture, varies so little as betweenadjacent apertures that it becomes important only cumulatively. Usabletubes can be constructed applying :a uniform correction throughout thescreen where the maximum angle of deflection of the beam is small; i.e., less than about 30". If the correction is to be made mechanically,however, tubes employing deflections of larger value can be corrected inzones, using phosphor strips of equal width for some fixed distanceoutward from the center of the screen, and strips of very slightly lesswidth in one or more successive zones. In a tube wherein the lens-screendistance is from half to three-quarters inch (500 to 750 mils) the totalcorrection in a tube wherein the maximum angle of deflection is 35 or 36degrees, may be of the order of l or 2 phosphor groups, the screen beingthat much narrower than one wherein the electrons were not refracted bypost-deflection focusing. A similar refraction effect takes place withinthe lens element here described. It is usually unnecessary that it becompen sated for,- as far as the conductors 23 and 25 areconcerned, fortwo reasons. One is that the total thickness of the lens structure isordinarily only 10% or less of the distance between the grid and screenof a twoelement lens of the type mentioned and therefore the effects tobe corrected for are only about 5% as great, as the second half of thelens partially corrects the refraction produced by the first half; thesecond is that the positioning of the conductors 23 and 25 is not nearlyas critical as the positioning of phosphors. A linear correction, withthe spaces between the conductors 23 and 25 respectively proportional todistance from the center of deflection, as mentioned, is thereforeentirely adequate.

The same factors which have been mentioned in connection with thespacing of the conductors 23 and 25 renders the correction of the screenpattern for refraction simpler and more accurate than the correction byZones, in the other type of lens mentioned. When the screen is operatedwithin even a few hundred volts of that of the electrode 25,substantially all of the refraction wiil take place within the. lensstructure itself, the paths of the electrons when they leave the lensstructure will be substantially rectilinear, and therefore a singleuniform correction can be applied to the screen, with a re-' sult evenmore accurate than is given with a zonal correction for a two-elementlens, using a single grid and a positive electrode on the screensurface. Furthermore, by operating the conductors 25 at a somewhathigher potcntial than conductors 23, thereby making the lens fieldsslightly asymmetrical, the refraction of the principal ray of the beamwithin the lens can be almost completely neutralized, so that thespacing of the phosphor groups may be in the same proportion to thespacing of the grid conductors as -their respective distances from thecenter of the screen.

While all of the figures showing field conformations have shown the gridconductors as wires, it will be recognized that the same principlesapply when the slat or tape formation illustrated in Fig. 3 is employed.With as wide strips as are shown in Fig. 3 the field conformations willbe slightly different, and electrodes 23 and 25 can be slightly nearerthe edges of the tapes as in the case where wires are used, but thedirection of the fields and their focusing etfects are identical. Thefocusing fields are, however, confined to the portion of the gridelements adjacent to their edges and therefore it will be seen that thetapes may degenerate into wires without changing the over-all effect asfar as the focusing function of the grid is concerned.

Fig. 4 shows another form of three-element electron lens and gridstructure having much the same effect but somewhat more difiicult toconstruct. In this case, conductors 33 and 35 comprise the elementswhich are mounted parallel to the strips of phosphor, whereas theelement 37 is a gauze or transverse conductors such as have beendescribed. As far as focusing fields are concerned, this structure alsogives the analog of a double convex lens, and the elements 35 will beoperated at the same potential or nearly the same potential as thescreen. Electrons, therefore, still follow rectilinear paths from thelens structure to the screen, and since the structure is symmetrical asfar as the focusing fields at the edges of conductors 33 and 35 areconcerned, the path of the ray entering the center of the aperture is acontinuation of or parallel to the path of the parting electrons. Thestructure in the preceding figures is preferred, however, because itdoes not require exact alinement between conductors 33 and 35.Mechanically, therefore, the structures previously described offer fewercomplications, although the effects produced may be made very nearly ifnot quite the same. Both this and the preceding structures have beenreferred to as analogous to double-convex lenses, but it would perhapsbe more strictly accurate to consider the structures first described asanalogous to two lano-convex optical lenses with their convex surfacesface-to-face, and the structure of Fig. 4 as analogus to a double convexlens, or two plane-convex lenses, back to back. The focusing effect issubstantially the same in either case. It will be noted that in thiscase, as in the others, the gauze or transverse conductors are operatedat a potential positive with respect to the other conductors of thestructure.

A somewhat less desirable but still completely operative structure isshown in Fig. 5. In this figure only the two elements are used toestablish the fields which constitute the lens and the latter istherefore equivalent to a planoconvex optical lens, and can beconsidered as constituting one-half of either the lens of Fig. 3 or thatof Fig. 4. If the screen is allowed to seek its own potential, andtherefore operates at substantially the potential of theapertureforrning electrodes 39, the paths of the electrons between the.grid and the screen will be substantially rectilinear as before.

Where permeable electrode 41 comprises either gauze or paralleltransverse wires, the structure does not possess the, flexibility ofoperation which the double-convex structures do, but is nonethelesscapable of giving satisfactory resuits. Being assymetrical, the emergentbeam is not parallel to the incident beam, and the correction of thescreenpattern for refraction will be substantially the same as withsingle-grid and film lenses if the screen, and the electrodev nearest itare operated at the same potential. This can be verylargely compensated,however, by applying a moderate potential difference between these twoelements to introduce an opposite refraction. If the aperture-forminggrid is nearer the screen the latter should be made: the more positiveelement, but if the electron-permeable electrode is the nearer, the.screen shouldv be negative to effect compensation of refraction.

In contrast to the structures. thus far described are those shown inFigs. 8, 9., and 1.0,. which are structures which have been suggestedfor the. over-all purpose of those above discussed. In Figs. 8 and 9 thestructures are the same, each comprising three grids composed ofelectron optically alined conductors 43, 45 and 47. The conductors ofthe respective grids are alined parallel to the electron paths, and thetwo figures dilfer in that in Fig. 8, the central grid is operated at anegative potential with respect to the other two Whereas in Fig. 9 thereverse is the case. In either of these arrangements focusing of theelectrons occurs. It will be seen from the lines indicating the fielddirections, the electrons passing near one edge of the aperture will besubjected to equal transversely accelerating forces in both directions.In Fig. 8 the electrons are traveling most rapidly as they pass betweenthe conductors 43 and are therefore subjected to the diverging componentfor a relatively short time. They are retarded as they approachelectrodes 45 and are subjected to a converging component and the lattertherefore has more time to act. so that the transverse velocitycomponent is reversed in direction. As they pass electrodes 47, they areagain subjected to a diverging field, but again they are traveling at agreater velocity and this field'has less effect. The net effect is aconvergence. The converging eifect of such a lens as this, however, is asecond order effect. The potentials required to produce a givenconvergence, in a practical tube, must be an order of magnitude greaterthan those necessary in lenses of the type to which this specificationis directed. In Fig. 9 the same general effects occur. Here the firstand last fields traversed are converging and are effective when theelectrons are traveling at their minimum velocity, while the electronsare traveling most rapidly when subjected to the diverging fieldstraversed as they pass near the middle set of electrodes 45. The sameremarks as apply to Fig. 8, with respect to necessary focusingpotentials, also apply to Fig. 9. In addition to the necessity forhigher focusing potential the structures require exact alinement betweenthe three grids.

In contrast to the structures in Figs. 8 and 9 is that of Fig. 10 whichis similar to the two previous figures with the exception that theelectrodes 43' and 47' are shifted by one-half of the width of theapertures formed between the conductors 45', these being the apertureswhich determine the alinement with respect to the phosphors on thescreen. Strictly speaking this is not a lens structure at all, becauseany electron entering it must cut all of the lines of force existingbetween three sets of electrodes and therefore will be deflected to anequal degree in one direction or the other. Electrons entering to leftof conductors 45 and to the right of conductors 43 will be deflected tothe left, while those entering the other side of the aperture betweenconductors 43 will be deflected to the right. If the potentials arecorrectly chosen a width of the beam as it strikes the screen may bereduced by one-half, the structure being the analog of a series ofparallel prisms. For some purposes this may be a desirable structure,and it will be seen that it conforms to the broad description of thepresent invention in that there are conducting elements lying within theapertures of the grid, as viewed along the electron paths, while thestructure of Figs. 8 and 9' do not. These latter structures areoperative with relatively low voltage tubes but where high electronvelocities are employed the outer electrode voltages required to producea given focal length are so high as to introduce danger" of breakdown byfield emission from the electrode elements. Additional conductors,parallel to electrodes 43" and 47' and in the same plane as conductors45' would make the arrangement conform more nearly to a true lens, theoptical analog being a multi-sided prism whose surfaces approximated thesurface of a cylinder. The disadvantage of such a structure, inintercepting a relatively large proportion of the beam as it passesthrough the ions, have already been discussed.

Where a single conductor parallel to each pair of grid" 1t the exactalinement between them does not have the same importance, but if theiralinement is not carefully calculated a further interception ofelectrons may occur.

With respect to the types of lenses to which this specification isparticularly concerned, the electrodes of the bi-convex types have beenshown as uniformly spaced and the potential differences between thegrids and the electron permeable-electrodes have been indicated asequal. Neither of these is a necessary condition. Just as bi-convexoptical lenses may have different curvatures of their two faces, andstill have the same focal length as a symmetrical lens of the samegeneral type, so may an electron lens. A given voltage between thelens-forming elements will produce the same total displacement of thebeam, irrespective of the separation of the elements of the lensstructure, but the electron-optical coefficient of refraction and hencethe thickness of the lens required to produce such a displacement of thebeam depend also on the separation of the elements to which the voltagedifferences are applied.

' Therefore, just as the aberrations of optical lenses can be largelycompensated by combining lenses of different focal length with differentcoefficients of refraction, so can errors of refraction be correctedwith electron lenses of more than two elements.

It is this fact which gives the structures here described theirflexibility. With a lens of the type shown in Figs. 2, 3, and 6, thesame focal length can be had, for a given velocity of incidentelectrons, if the arithmetic sum of the voltages between the grid 21 andelectrodes 23 and 25 respectively is a constant, but the refraction,which determines the positions onv the screen at which the focused beamswill impact, depends on how the voltages contributing to the sum aredivided between the two. The voltage relationship between the finalelement of the lensgrid structure gives an additional degree of freedomof design. Therefore such structures as are here described not onlyrequire smaller corrections but they permit larger manufacturingtolerances, since adjustments in the computed corrections can be madeafter a tube embodying the structures is completed.

structurally, numerous ways of constructing grids of the general typehere considered have been proposed and used. This invention, however,does not relate to the mechanical structure but to the disposition ofthe conductors relative to each other, and particularly to theprinciples which lead to such disposition. Such details as have beenshown and described are therefore not to be considered as limiting thescope of the invention, all

intended limitations being expressed in the following claims.

, I claim:

1. In a cathode-ray tube for displaying television images in color,including means for projecting electrons in modulated beam conformationagainst a target area across which said electrons are adapted to bedeflected to trace a raster, target structure Within said areacomprising a screen having a display surface of phosphors emissive onelectron impact of light of different colors additive to produce whitelight and arranged in a repeating pattern of groups of generallyparallel strips, each group including at least one strip emissive ofeach of said colors, a grid of elongated linear conductors mountedadjacent to said screen with said conductors substantially parallel tosaid strips and with each pair of adjacent conductors defining anaperture which is electron-optically alined with a corresponding groupof strips, a pair of electronpermeable electrodes mounted respectivelyon each side of said grid and substantially coextensive with said gridand each including conducting elements within the area defined by eachof said apertures as viewed along the paths of electrons projectedtherethrough, and connections for applying diiferent potentials to saidgrid and said electrodes respectively.

2. The combination as defined in claim 1 wherein each tion for applyingan electrical potential thereto.

5. In a cathode-ray tube for displaying television images in color,including a plurality of electron guns positioned to project a likenumber of electron beams converging on a target area across which saidbeams are adapted to be concurrently deflected to trace a raster, targetstructure within said area comprising a screen having a display surfaceof phosphors emissive on electron impact of light of different colorsadditive to produce white light and arranged in a repeating pattern ofgroups of generally parallel strips, each group including at least onestrip emissive of each of said colors, a grid of elongated linearconductors mounted adjacent to said screen with said conductorssubstantially parallel to said strips and with each pair of adjacentconductors defining an aperture which is electron-optically alined witha corresponding group of strips, a pair of electron-permeable electrodesmounted respectively on each side of said grid and substantiallycoextensive with said grid and each including conducting elements withinthe area defined by each of said apertures as viewed along the paths ofelectrons projected therethrough, and connections for applying differentpotentials to said grid and said electrodes respectively.

6. In a cathode-ray tube for the display of television images in colorwhich includes means for projecting electrons in modulated beamconformation against a display screen having a surface of phosphors,emissive on electron impact of different color light arranged in arepeating pattern of groups of substantially parallel strips, each groupincluding at least one strip emissive of each color and the width ofeach group being of the order of magnitude of one elemental area of theimages to be displayed, a focusing structure substantially coextensivein area with said screen mounted adjacent thereto and comprising a gridof tightly stretched linear conductors mounted substantially parallel tosaid phosphor strips to define an aperture between each pair of adjacentconductors which is electron-optically alined with a corresponding groupof phosphor strips and an electron-permeable electrode comprisingconductors extending in a direction transverse to said grid conductorsmounted generally parallel to said grid, said grid being materiallycloser to said electrode than to said screen, and connections forapplying different potentials to said grid and said electrode.

7. Ina cathode-ray tube for the display of television images in colorwhich includes means for projecting electrons in modulated beamconformation against a display screen having a surface of phosphors,emissive on electron impact of different color light arranged in arepeating pattern of groups of substantially parallel strips, each groupincluding at least one strip emissive of each color and the width ofeach group being of the order of magnitude of one elemental area of theimages to be displayed, a focusing structure substantially coextensivein area with said screen mounted adjacent thereto and comprising threegenerally parallel electrodes, at least one of said electrodescomprising a grid of tightly stretched linear conductors extending in adirection substantially parallel to said phosphor strips, each adjacentpair of said conductorsdefining an aperture electron-optically alinedwith a corresponding one of said strips, a second of said electrodescomprising conductors extending transversely of 'said apertures'and thethird of said electrodes comprising conductors extending in a directionsubstantially parallel to the conductors of one of the other two of saidelectrodes, and connections for applying different potentials to each ofsaid electrodes, the spacing between said electrodes being materiallyless than the spacing between said grid and said screen and theconductors of the center electrode extending in a direction transverseto the conductors of the two outer electrodes of the structure.

8. In a cathode-ray tube for the display of television images in cOlorwhich includes means for projecting electrons in modulated beamconformation against a display screen having a surface of phosphors,emissive on electron impact of different color light arranged in arepeating pattern of groups of substantially parallel strips, each groupincluding at least one strip emissive of each color and the width ofeach group being of the order of magnitude of one elemental area of theimages to be displayed, a focusing structure substantially coextensivein area with said screen mounted adjacent thereto and comprising threegenerally parallel electrodes, at least one of said electrodescomprising a grid of tightly stretched linear conductors extending in adirection substantially parallel to said phosphor strips, each adjacentpair of said conductors defining an aperture electron-optically alinedwith a corresponding one of said strips, a second of said electrodescomprising conductors extending transversely of said apertures and beingmore closely spaced than said grid conductors, and the third of saidelectrodes comprising conductors extending in a direction substantiallyparallel to the conductors of one of the other two of said electrodes,and connections for applying diiferent potentials to each of saidelectrodes, the spacing between said electrodes being materially lessthan the spacing between said grid and said screen and the conductors ofthe center electrode extending in a direction transverse to theconductors of the two outer electrodes of the struc-' ture.

9. In a cathode-ray tube for the display of television images in colorwhich includes means for projecting electrons in modulated beamconformation against a display screen having a surface of phosphors,emissive on electron impact of different color light, arranged in arepeating pattern of groups of substantially parallel strips, each groupincluding at least one strip emissive of each color and the width ofeach group being of the order of magnitude of one elemental area of theimages to be displayed, a focusing structure substantially coextensivein area with said screen mounted adjacent thereto and comprising a gridof tightly stretched linear conductors mounted substantially parallel tosaid phosphor strips to define an aperture between each pair of adjacentconductors which is electron-optically alined with a corresponding groupof phosphor strips, a pair of electron-permeable electrodes positionedrespectively on each side of said grid and comprising a grid ofelongated linear conductors extending transversely of said apertures andmore closely spaced than the conductors of said first-mentioned grid theconductors of said electron-permeable electrodes being positioned instaggered relationship as viewed along the electron paths through saidapertures.

References Cited in the file of this patent UNITED STATES PATENTS Re.23,672 Okolicsanyi June 23, 1953 2,172,845 Holzer Sept. 12, 19392,213,547 Iams Sept. 3, 1940 2,228,978 Schade Jan. 14, 1941 2,669,675Lawrence Feb. 16, 1954 2,692,532 Lawrence Oct. 26, 1954

